Extraction of alumina from ores



May 8, v1945.

R. w. BROWN EXTRACTION OF AT IUMINA FROM CRES Filed May 1s, 1942 'least,with existing equipment.

Patented May 8, 1945 EXTRACTION OF ALUMINA FROM O'RES Ralph Waldo Brown,East St. Louis, IllL, assigner to Aluminum Company of America,Pittsburgh Pa., a corporation of PennsylvaniaA Application May 18, 1942,Serial No. 443,402 3 claims. (ci. A2 3-141) This invention relates tomethods of recoveringi``f"` 'alumina of high purity from low gradealuminabearing ore. Heretofore, the Bayer process, or modificationsthereof, has been-used for the recovery, from alumina-bearing ore, ofmost of the high purity alumina used in the production of aluminum. TheBayer process consists in first pressure-digesting a slurryl of bauxiteand caustic liquor to form an alumina-rich solution. This solution isthen separated from its accompanying A' residue which is called red mudand consists 4 of undissolved portions of the bauxite as well as gradeores above described is effected simultaneously and in conjunction withoperation of the Bayer-type process on bauxite, preferably lowf 4silicabauxite. As a. first and preliminarystep,

5 the low grade ore is treated to solubilize its alul 'mina values andto separate these values from the silica and other unwanted impuritiesin the ore. To thisend the red mud, clay or other silicious ore is mixedwith proportioned quantim ties of alkaline earth compound and alkalimetal insolubles formed during digestion. This red mud is discarded asWaste material. The alumina-rich solution is then seeded with freshlyprecipitated alu-mina and agitated to precipitate alumina as aluminumhydrate. After separationi from the hydrate thus(precipitated, the spentorm mother liquor is returned tothe pressure diges-`- tion may beeffected upon subsequent leaching of tion step where its caustic valuesare again utilized in the treatment of more bauxite. Successfulcommercial operation o f this Bayer process is confined tothe treatmentof `bauxite containing less than about 6 per cent by weight of silica.Bauxites of higher silica content raise the cost and decrease theelliciency of the process. Clays, and other ores bearing alumina values,usually in silicate form, are not successfully treated Iby the Bayerprocess. Among the other low grade ores not susceptible of suchtreatment is the aforesaid red mud which contains insolubilized aluminaand caustic values. Years of operation of the Bayer process haveaccumulated large stocks of this waste material.

Many processes of various'types have been proposed for the commercialextraction of alumina from these low grade alumina-bearing ores, :butnone have been commercially operated vto successfully produce highpurity alumina at a commercially acceptable cost. Accordingly, mostpresent commercial installations available for the recovery of aluminaoperate on the principle of the Bayer process and are, therefore,unsuitable for the treatment of low grade ores by any process hithertoproposed.

Among the objects of this invention is the provision of a method ofobtaining high purity alumina from low grade alumina-bearing ores whichcan be operated at comparatively low cost and which may, if desired, beoperated', in part at Other objects will appear in the followingdescription of the invention.

In' the operation of the methods of this invention, recovery of aluminavalues from the low compound and 4then sintered. Preparatory to mixing,these materials, including the ores.

should Ibe ground or pulverized, if necessary, to particles of such sizeas will promote complete n ,5- mixing and elfectivev reaction duringsintering.

Theproportions of the mixture are adjusted to promote the purpose of thesintering which is to insolubilize impurities and to solubilize aluminaand other wanted values so that a separathe sintered mass. The vfunctionof the alkaline earth compound is to insolubilize the silica. For thispurpose any such compound which forms, on reaction, insoluble silicatesmay be used, but

the readily available and low cost limestone is preferred. The amount tobe added depend l upon the amount of silica in the ore. If, as in thecase of some red mud, the ore initially contains alkaline earthcompound, the amountnecessarily added may be correspondingly reduced. At

least a slight excess is desirable in the nal mixture. For instance, iflimestone be used, the amount present should be somewhat in excess ofthat required to furnish Sucient CaO to form,

with the silica, the insoluble dicalcium silicate (2CaO.SiOz) The alkalimetal compound should y.be added to the mixture inamount suflicient tofurnish at least a slight excess of available alkali metal overthatrequired to form soluble alkali metal aluminate with the alumina contentof the ore. Some cheap source of alkali metal is preferred and theplentiful and cheap soda ash is usually employed. Desirability of theuse of larger amounts of alkaline earth compound and alkali metalcompound than those above indiand alkali metal compound, proportioned asabove described, is subjected to a sintering operation at suchtemperatures as to cause fritting or sintering of the particles'withoutfusion or melting of. large portions of the mixture. Since melting ofsubstantial portions of the mass would interstantial portion of theseare also released in soluble form and made available for subsequentrecovery by leaching. l

Sintering being complete, the sinter is then leached for the purpose ofrecovering therefrom the soluble alumina and caustic values. Water maybe used as the leaching medium as may also solutions containing causticvalues, These latter may be obtained, if desired, as a by-product of theoperation of the Bayer process as hereinafter described. In any event,the result of the leaching is a leach liquor containing most of thealumina values solubilized by the sintering operation. This liquor willalso contain, however, quantities of. solubilized impurities,principally the desired alumina values and the unwanted silica values ofthe ore. To hasten the leaching process the leaching media may beheated, but

preferably to temperatures not much in excess of 200 F. since highertemperatures promote formation of insoluble complexes containing aluminavalues. The residual insoluble portions of the sinter, which now consistprincipally of alkaline earth silicate as well as other insolu-` bilizedimpurities plus some alumina values, are discarded. The leach liquor,containing much of the desirable values of the original ore, is nowready for further treatment in accordance with the principles of thisinvention.

As above mentioned the leach liquor contains substantial amounts ofsoluble silica, The precipitation processes previously known andproposed for the purpose of recovering the alumina content of thisliquor are not capable of precipitating all the available aluminawithout also precipitating the silica. Since the quantity of silicapresent in the leach liquor is usually about 2 per cent or more byweight of the available alumina, the result is an alumina precipitatecontaining much more silica than is acceptable in a high purity alumina.Production of metallic aluminum of desired commercialpurity requires analumina containing less than 0.1 per cent of silica and usually lessthan 0.06 per cent. The problem is, therefore, one o'f obtaining such analumina from the leach liquor.v

The present invention meets this problem in the silica. This reects theinability of the sintering operation to eiect complete separationbetween handling of the solution and the precipitation of aluminatherefrom by the auto-precipitation process? When the caustic is presentas sodium hydroxide (the usual form), the ratio usually desired,expressed as A1203 divided by the total NaOH present (including thatcombined with the alumina), is from about 0.8 to about 1. This slurry isthen digested in the usual manner at steam pressures of about 60 to 200pounds per square inch. The alumina-rich solution formed by thisdigestion is separated from insolubles by ltering, sedimentation, orlike processes, and the insolubles, after being washedv to removeentrained caustic and alumina values, are discarded. The alumina-richsolution is then treated to precipitate its alumina values.Auto-precipitation is accomplished by the well-known methods of seedingand agitation such as those described by Bayer (U. S. Patent No.382,505), Sherwin (U. S. Patent No. 1,251,296), and Fickes (U. S.Reissue Patent No. 13,668). The precipitate thus obtained is of highpurity containing less than about 0.06 per cent by weight of silica. Itis washed to remove residual caustic values and as washed is the endAproduct .of the process.

`The mother liquor resulting from the precipitation is composed of mostof the caustic values and the unprecipitated alumina values. It isreturned, at least in part, to the digestion step where it furnishes thecaustic liquor necessary t0 the digestion of the next batch of bauxite.There it returns its residual alumina values to the process. A portionof this mother liquor may be diverted to serve as leaching media in theleaching of the low grade ore sinter. Similarly the alkaline wash watersresulting from the -washing operation above described may be used in theleaching process or may be concentrated for use in the digestion step.

The introduction of the leach liquor from the low grade ore process intothe Bayer process at the point where the bauxite-caustic liquor slurryis formed overcomes the difculties previously encountered in obtaining ahigh purity alumina precipitate from such leach liquor. The subsequentsteps of the Bayer process eliminate the silica with the result that thenally precipitated alumina is substantially silica-free. This result isachieved with the simultaneous recovery of similar alumina from bauxite,and thus existing equipment is utilized -to perform, at low cost, a dualfunction. Other and additional advantages i'low from the process hereindescribed and manner now described. As above mentioned the inventioncontemplates simultaneous operation of the sintering and leaching of lowgrade ores and of a Bayer process operating, as previously described. onbauxite. It is the gist of the invention that these two processes aretied together by transferring the leach liquor` resulting from the lowgrade ore process to the Bayer process where the leach liquor is addedto, and becomes a part of, the bauxite-caustic liquor slurry formedprior to the digestion step, The resultant slurry contains, therefore,not only the bauxite and the caustic liquor of the usual Bayer processslurry, but also the alumina and caustic values of the leach liquor.The-ingredients of this slurry are so adjustedthat the ratio of aluminato caustic in the alumina-rich solution produced by the sub-y sequentdigestion will be suitable to the later claimed. 'I'he Bayer process asnormally practiced is a cyclic process in which the caustic valuesoriginally introduced into the' process to dissolve alumina values frombauxite are recovered, after precipitation of alumina from thealumina-rich solution formed by the digestion, and returned to theslurry wherev again they are used to treat bauxite. Some loss of causticvalues takes place, however, during each cycle and replacement thereofis a material element of operation cost. In the practice of the presentinvention these required replacement values are furnished to the processby the introduction into the slurry of the leach liquor from the lowgrade lore process, thus obviating the necessity of separate causticreplenishment. Moreover, these caustic values are obtained during thesintering operation from crude and cheap alkali compounds, thusmaterially reducing the caustic costs of the Bayer process.

Therefore, by the improved processes of this invention, it is possibleto commercially produce high purity alumina from low grade ores, tosimultaneously reduce the cost of extracting alumina from bauxite, andto accomplish this total result by utilization, in part, of existingequipment'and facilities. A

The accompanying drawing is a ilow sheet illustrative of the process.

What I claim is:

1. In a process of extracting alumina of high purity from low gradealumina-bearing ore, including sintering of the ore with quantities ofalkaline earth compound and alkali metal compound proportioned tosolubilize the alumina values thereof and leaching the sinter thusformed to produce leach liquor Vcontaining soluble alumina values, theimprovement comprising forming a slurry composed of bauxite, causticliquor, and said leach liquor in such proportioned amounts as will ondigestion form an aluminarich solution having auto-precipitatingcharacteristics, digesting said slurry under pressure to form analumina-rich solution and thereafter precipitating alumina values fromsaid solution.v

2. In a combinationprocess of extracting high purity alumina from lowgrade alumina-bearing ore and oi extracting similar alumina from bauxitewhich includes sintering low grade ore with amounts of alkaline earthand alkali metal compounds proportioned to solubilize the alumina valuesof said ore and leaching the resultant sinter to form leach liquorcontaining soluble alumina values and which further and separatelyincludes pressure digestion of a slurry of bauxite and caustic liquor toform an alumina-rich soluitating alumina from" said solution, separatingthe mother liquor from the precipitate and resolution having tion,separation of said solution from insolubles and precipitation of aluminafrom said solution. the improvement consisting of adding'said leachliquor to said slurry prior to said digestion whereby the aluminavvalues of the leach liquor are included in the said alumina-richsolution, the components of the iinal slurry being so propor-v tioned asto produce, on said pressure digestion, an alumina-rich solution havingauto-precipitating characteristics.

3. In a cyclic process of extracting alumina from bauxite whichcomprises forming a slurry of bauxite and caustic` liquor,pressure-digesting said slurry to form an alumina-rich solution,separating said solution from insolubles, precipturning the motherliquorto lthe slurry to provide further caustic liquor for the treatment ofadditional bauxite, the improvement consisting of sintering low gradealumina-bearing ore with quantities of alkaline earth and alkali metalcompounds proportioned to solubilize the alumina values of said ore asalkali metal aluminate, leaching the resultant sinter to form a leachliquor containing soluble alkali metal aluminate and adding Isaid leachliquor'to said slurry of bauxite and caustic liquor, the components ofthe nal slurry being so proportioned as to produce, on said pressuredigestion, an alumina-rich auto-precipitating character- RALPH WALDOBROWN.

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